In internal combustion engines, when the combustion temperatures exceed about 2,500° F. (about 1,371° C.), atmospheric nitrogen begins to react with intake oxygen to form compounds called nitrogen oxides (NOx). These compounds play a major role in air pollution, particularly in high traffic areas such as cities. To reduce the formation of NOx, combustion temperatures must be kept below the 2,500° F. threshold. One way this is achieved is by re-circulating a small amount of engine exhaust back into the engine intake through an exhaust gas recirculation (EGR) valve.
The EGR valve controls a passageway between the intake and exhaust manifolds. Sensors and a control unit are used to open the valve to allow the intake vacuum to draw a portion of the exhaust stream through the valve and into the intake stream. The exhaust gas flow dilutes the incoming air/fuel mixture and has a quenching effect on combustion temperatures, which keeps NOx production within acceptable limits. As an added benefit, it also reduces the engine's octane requirements which lessens the occurrence of detonation (spark knock).
However, when a waste heat recovery (WHR) system is also used to recover energy from the EGR exhaust flow before it enters the engine intake system, condensation formation at the engine intake can pose a problem. Condensation on the intake system may damage the engine by breaking down oil film on engine cylinder surfaces and by clogging intake ports as it combines with combustion soot to form wet soot deposits. The main cause of this condensation is an EGR flow outlet temperature which is too low as a result of the temperature of the working fluid in the WHR system being much lower than the engine coolant temperature.
The disclosed system and methods address these problems in the prior art by providing control of the EGR outlet temperature. The system provides means for protecting against cooling the EGR stream beyond a threshold which would cause condensation. Likewise, the methods provide for controlling the EGR outlet temperature and protecting an engine from damage due to heavy condensation at the intake system.